Gratings

Michelle contains 5 gratings to provide various spectral resolutions in the 10 and 20 micron atmospheric windows. Characteristics of the gratings are shown in the table below. The resolving powers of the gratings are proportional to wavelength across each window (i.e. the resolution is constant in microns), for each slit; values listed are for 10um and 20um. The resolving power of the echelle oscillates with wavelength, depending on the angle of the echelle. Resolving powers vary inversely with the slit width (e.g., R with the 3-pw slit is 2/3 that with the 2-pw slit). Because the echelle is in low order, resolution and efficiency vary rapidly with wavelength. The echelle blaze angle corresponds to wavelengths of 7.4, 8.2, 9.3, 10.6, 12.3, 14.8, 18.5, and 24.7um. Close to these wavelengths the resolution and sensitivity are roughly 30,000 with the 2-pw slit and 20,000 with the 3-pw slit. Midway between these
wavelengths efficiency is much lower and the resolving power can be 1.5 times higher or lower. Until additional order sorting filters for parts of the Q window are obtained, the echelle cannot be used beyond 22.5um or at the wavelength of the H2 0-0 S(1) line (17.04um)

Name

Usable wavelengths

Dispersion (microns/pixel)

Resolving power
10um, 2pw slit

Resolving power
20um, 3pw slit

Wavelength coverage for single grating setting (microns)

LowN

7-14um

0.024

200

-

7.7

LowQ

16-26um

0.031

-

110

9.9

Medium (R~1000)

7-26um

0.0047

1000

1300

1.5

High (R~3000)

7-26um

0.0016

3000

4000

0.50

Echelle

7-22.5um *

~ 20,000-40,000

~ 13,500-27,000

1500 km/sec at blaze wavelength

* The echelle cannot be used in the following wavelength ranges: <7.33um, 8.13-8.33um, 13.10-17.10um, and >22.5um - no suitable order blocking filter;
13.33-13.48um, 16.00-16.45um, and 20.00-20.69um - echelle angle exceeds 75 deg.